The present invention relates to a bolt tightening structure of a magnesium alloy members, and more particularly to a technology for preventing occurrence of electrolytic corrosion of the magnesium alloy member.
Recently, in the automobile industry, as the concern about environmental problems increases, a further improvement of fuel economy has been demanded. To meet such demand, in the automobile industry, it is required to reduce the weight of the car body, and magnesium alloys are widely used in automotive parts owing to their lightest weight among practical metals. More lately, in particular, they are applied in locations where very high corrosion resistance is required such as exterior and structural parts.
However, since the magnesium alloy is the basest practical alloy, when it is tightened with a heterogeneous metal such as iron or aluminum, electrolytic corrosion is likely to occur in the presence of moisture containing electrolyte. Especially for automobiles used outdoors, electrolytic corrosion is extremely promoted by the function of electrolyte contained in rainwater, seawater, or snow-melting salt, possibly causing breakage or damage of parts.
Japanese Laid-open Patent No. 5-302614 proposes a technology for preventing electrolytic corrosion by interposing a paper washer coated with resin between a magnesium alloy member and a steel bolt so as to avoid contact between the bolt and the magnesium alloy member. However, as the bolt corrodes, corrosion products flow out onto the magnesium alloy to cause electrolytic corrosion, and therefore it was impossible to apply in the position where corrosion resistance was required. It may be also considered to insulate between a bolt head and a magnesium alloy member by means of a resin washer, but the resin creeps by heat in coating, and the bolt shaft power lowers, and retightening is required, and hence it is not practicable. Moreover, recently, automakers in Europe have developed techniques for interposing an organic material containing inorganic fillers between a magnesium alloy member and a bolt. In such techniques, however, after 20 cycles of composite corrosion test by salt spray, occurrence of electrolytic corrosion was noted, and it was far from satisfaction of recent demands.